1. Field of the Invention
The present invention relates to a condenser microphone which can adjust variation in the sensitivity between individuals without changing an electrostatic absorption force between a diaphragm and a fixed pole.
2. Description of Related Art
A condenser microphone has a diaphragm and a fixed pole which oppose to each other with a predetermined gap, and outputs as an acoustic signal a change in an electrostatic capacity between the diaphragm and the fixed pole due to vibration of the diaphragm upon reception of an acoustic wave. There is a direct current bias condenser microphone which applies a direct current voltage to a condenser formed with a diaphragm and a fixed pole. The sensitivity of the direct current bias condenser microphone depends on a bias voltage, and, when the bias voltage is higher, the sensitivity is higher. Hence, there is a direct current bias condenser microphone which can adjust the sensitivity by adjusting a bias voltage. FIG. 6 illustrates an example of a circuit diagram of a condenser microphone of a relevant technique.
In FIG. 6, the condenser microphone unit 15 has a diaphragm 151 and a fixed pole 152 which oppose to each other with an adequate gap, and, when the diaphragm 151 receives an acoustic wave and vibrates, the acoustic wave is electroacoustically converted as described above and is outputted as acoustic signals from the diaphragm 151 and the fixed pole 152. The fixed pole 152 is connected to a direct current voltage source 16. Further, the diaphragm 151 is connected to a non-inverting input terminal of an amplifier 11. The amplifier 11 has an impedance converter formed with a FET (Field Effect Transistor), and converts an output signal of a condenser microphone unit 15 of a very high impedance into a low impedance signal and outputs the signal to the next stage.
An output terminal of the amplifier 11 is connected to an inverting input terminal of an inverting amplifier 12 which is a second amplifier, through an input resistance R10. A non-inverting input terminal of the inverting amplifier 12 is connected to an earth through a condenser C12. A feedback resistance R11 is connected between the inverting input terminal and the output terminal of the inverting amplifier 12. A resistance value of the input resistance R10 and a resistance value of the feedback resistance R11 are equal. Here, a voltage gain of the inverting amplifier 12 is −1.
The output terminal of the amplifier 11 is connected to a non-inverting input terminal of a third amplifier 13. The output terminal of the inverting amplifier 12 is connected to a non-inverting input terminal of a fourth amplifier 14. The output terminal of the amplifier 13 is connected to a second pin 22 of a connector employing a three-pin configuration. Further, the output terminal of the amplifier 13 is connected to the inverting input terminal of the amplifier 13. An output terminal of the amplifier 14 is connected to a third pin 23 of the connector, and is connected to the inverting input terminal of the amplifier 14. The connector is a connector for a balanced output. That is, the second pin 22 connected to the output terminal of the amplifier 13 is a hot-side output pin of a balanced output, the third pin 23 connected to the output terminal of the amplifier 14 is a cold-side output pin of a balanced output, and the first pin 21 is an earth pin.
A positive power supply terminal of each amplifier 11, 12, 13 and 14 is connected to a positive power supply output terminal of a direct current power supply 16, and a negative power supply terminal of each amplifier 11, 12, 13 and 14, an earth terminal of the direct current power supply 16 and the first pin 21 of the connector are connected to each other. Further, the direct current voltage outputted from the direct current power supply 16 is connected so as to be applied to the diaphragm of the condenser microphone unit 15 as a direct current bias voltage. The direct current bias voltage can be adjusted by operating a variable resistor VR11 which the direct current power supply 16 has. The variable resistor VR11 is configured to be capable of adjusting the sensitivity of the condenser microphone unit 15 upon this adjustment.
Thus, the direct current bias condenser microphone can adjust the sensitivity by adjusting the direct current bias voltage. However, with the direct bias condenser microphone, when a bias voltage is increased, an electrostatic absorption force produced between a diaphragm and a fixed pole increases, and the diaphragm is absorbed to the fixed pole. Hence, the diaphragm cannot vibrate, and therefore the direct current bias condenser microphone does not function as a microphone in some cases.
Further, with the direct current bias condenser microphone, even when the diaphragm is not electrostatically absorbed to the fixed pole by the bias voltage, if a strong air current hits the diaphragm, the diaphragm is more likely to be electrostatically absorbed to the fixed pole. Hence, by taking stability of an operation into account, a bias voltage is set at such a level that electrostatic absorption does not occur even if a strong air current hits the diaphragm. If it is possible to make the direct bias voltage continuously variable, it is possible to make the sensitivity of the condenser microphone continuously variable. By so doing, the direct current condenser microphone can set an adequate bias voltage at which the direct current condenser microphone can stably operate without electrostatic absorption even if a strong air current hits the diaphragm.
For example, an electret condenser microphone in which an electret layer is formed on the surface of a fixed pole does not need to be applied a direct current bias, and does not need a direct current bias generating circuit. However, in order to make the sensitivity of the electret condenser microphone, a configuration needs to be employed where the electret layer on the surface of the fixed pole is exposed, and a surface voltage of the electret layer is adjusted.
The applicant of the present invention proposed an electret condenser microphone which can adjust the sensitivity by making a direct current voltage to be applied variable (see Japanese Patent Application Laid-Open No. 2006-295357). Although the electret condenser microphone disclosed in Japanese Patent Application Laid-Open No. 2006-295357 can adjust the sensitivity, when a direct current voltage to be applied is increased too high, the diaphragm is more likely to be electrostatically absorbed to the fixed pole, and thus there is a limitation for increasing the sensitivity. Therefore, it is desired that a condenser microphone which can adjust the sensitivity without changing the voltage between the diaphragm and the fixed pole is realized.
Incidentally, condenser microphones have variation in the sensitivity per individual, and quality of condenser microphones is made uniform in sensitivity adjusting process in manufacturing process. The direct current bias condenser microphone can remove the variation in the sensitivity by adjusting the direct current bias voltage. However, adjusting the direct current bias voltage involves the above problem, and there is also a problem that frequency response characteristics fluctuate. Hence, it is desired that a condenser microphone which can adjust the sensitivity without changing a polarization potential upon adjustment for removing the sensitivity per individual is realized.